I think it's a great find, never seen anything like it. Probably didn't take off and become popular because it would be very easy to put a pry-bar int the gap and spring the lock. If the patent date is in the 1870's then I'd expect the lock would have been manufactured probably before the turn of the century.
"GOOD LUCK". 2 7/8" high x 2 1/16" wide. Brass case. Brass shackle. "BARNES MFG. CO." is stamped on the obverse of the shackle. "PAT'D JUNE 24, 1879" is stamped on the reverse of the shackle. "BELLAMY'S PAT. JUNE 24 79" is stamped on the shackle of earlier locks. Mfg by: Barnes Mfg. Co. Charles L. Bellamy Patent # D11256.